This invention relates to powder metallurgy. More particularly, it relates to articles made by sintering particulate metal and to methods and compositions for producing such articles. In one important specific aspect, it is directed to improvements in loose or gravity sintering of particulate aluminum, for production of porous aluminum filters and the like. The term "aluminum," as herein used, embraces aluminum metal and alloys thereof.
In conventional powder metallurgical operations, a mass or body of particulate metal is compacted into a desired shape under substantial pressure and is then sintered to effect metal-to-metal bonding of the particles, thereby providing an article having useful structural strength. Sometimes, however, the compacting step is performed with minimal pressure or even entirely omitted, in order to produce an article of low density and high porosity such as a filter. Thus, in so-called loose or gravity sintering, mold or cavity is filled with uncompacted metal powder which is heated to sintering temperature in the cavity.
Articles have been produced by powder metallurgy from a variety of metals. Some of these metals, such as aluminum, present problems in that particles of the metal tend to bear an oxide coating that inhibits desired bonding in the sintering step. Other materials which, in particulate state, also tend to have refractory oxide surface layers (i.e., oxide coatings not readily reducible by conventional means) include titanium, chromium, silicon, manganese, beryllium, zirconium, and zinc. Stated with reference to aluminum (which has special commercial importance for various powder metallurgical applications), when high-pressure compacting is employed prior to sintering, the oxide coatings may be ruptured sufficiently to provide localized metal-to-metal contact; diffusion at these contact sites, and further bread-up of the oxide skin by the liquid phase formed by alloying elements during sintering, may then enable attainment of satisfactory bonding. If compacting pressure is low or absent (as in the case of loose sintering, i.e., to produce a filter or other high-porosity article), however, the oxide coating of the particles is not broken. In addition, the particles in such instances are more uniform in size than they are when a high-density article is to be produced; consequently the contact points are reduced, with resultant decrease in diffusion. For these reasons, then, difficulties have heretofore been encountered in efforts to produce highly porous articles by powder metallurgy from metals such as aluminum.